Procollagen is defined as the intracellular precursor of collagen. Since there are at least four types of collagen, there are also four types of procollagens. The immediate goal of this project is to establish their distribution in the components of the rat incisor tooth. Later, their distribution would be examined in the rest of the body. The procedure used is to prepare antibodies against each procollagen, allow them to bind to sections of the incisor tooth, and localize the binding sites by two methods: a) the commonly utilized peroxidase - antiperoxidase technique; and b) to label the antibodies beforehand with radioiodine and detect them by radioautography of the sections. So far results have been obtained with antisera against types I and III procollagens. With the antiprocollagen I serum, a most intense reaction is observed in predentin with a decreasing intensity gradient from the odontoblast surface to the predentin-dentin junction; the odontoblasts give a strong reaction. The periodontal tissue shows a strong reaction and the pulp, a moderate one. With the anti-procollagen III serum, the periodontal tissue reacts strongly and the pulp moderately, with a strong reaction on the walls of the pulpal capillaries. In conclusion the significant features of this study are: a) the decreasing gradient of reactivity for procollagen I in predentin corresponding to an increasing gradient in the amount of collagen fibers and suggesting a progressive transformation of the procollagens into collagens, as the odontoblasts continually deposit new predentin materials; b) a reaction for type II procollagen on the walls of pulpal capillaries; c) the active production of the two types of collagen in periodontal tissue and pulp.